Chapter 6 Conclusion and Future Work
6.1 Conclusion
The field sequential color liquid crystal display (FSC-LCD) has emerged as a new branch of LCD application. Sequential driving LED backlight represents a potential technological breakthrough in terms of optical efficiency. Because of its needless color filter and low material cost, the FSC-LCD has become the key technology for reducing the power dissipation and the resource consumption.
FSC-LCDs rapidly flash the primaries time-sequentially such that the colors are mixed by means of temporal integration in the eye. The lacking sub-pixels and color filters result in high transitivity and large aperture ratio. The primary chromaticities are determined solely by the LEDs which enable wider gamut and scalable number of primaries. Furthermore, the impulse driving of the backlight ensures high moving image quality.
However, color breakup (CBU) is the most disturbing artifact, which occurs in FSC-LCDs. The CBU reveals itself in the appearance of multiple color images of stationary object during saccadic eye motion, or along the edges of moving objects when tracking the objects with the eye. Although increasing the frame to several thousand Hz can completely eliminate CBU [94], it is highly unlikely that affordable panels with frame rates higher than 240 Hz will be widely available in the foreseeable future.
In this thesis work, we have successfully relaxed the requirements on 240 Hz frame rate and reduced CBU significantly by modulating the backlight. The proposed methods, such as color field arrangement and gray level redistribution, were implemented in the timing controller circuit. Additionally, the index of color difference has been utilized to evaluate the CBU. Compared to the CBU perception with previously sequential driving methods, the newly developed methods have greatly improved the image performance of FSC-LCDs. Tab. 6-1 shows the dynamic control algorithm is the first sequential driving LCD in the world adapted by image contents. For LC availability, field processing loading, and power consumption, this adaptation method also presents its competitiveness.
Tab. 6-1 The comparison between normal RGB, 3 prior arts described in Chapter1, and the proposed adaptation method
6.1.1 The consistent color fields in mobile-sized FSC-LCDs
On a 5.6-in LCD platform, we have demonstrated the color field arrangement (CFA) method, which modifies the consecutive color order to resolve CBU issue. The integration of three consecutive frames on retina was compensated as a gray level image because of the viewpoint through the same ratio of each primary color. Blurring margins of test patterns were observed in the synchronized camera experiment for the movement of eyes tracking. Comparatively, the perceived images with the typical 3-field RGB driving strategy have obvious multicolor margins, the notable CBU phenomenon.
The 4-CFA method with consistent orders of RGBR, GRBG, and BRGB in three consecutive frames was implemented and confirmed experimentally on the CBU suppression. The physical evaluation results on dynamic CBU adhere to the prediction for the moving images. Furthermore, the field frequency of CFA method is 1.3 times faster than that of typical one, resulting in the slighter static CBU for stationary images. Generally, increasing field frequency can effectively mitigate the static CBU.
However, the LC response time is still a crucial limitation to apply the sequential driving on LCDs.
We presented the 4-field RGBW method by flashing a gray image in a white field and displaying the color residuals in remaining fields. This allows the image energy to be better focused in the initial field, thus reducing the intensities of the red, green, and blue fields thus suppressing CBU within the field frequency of 240 Hz.
The minimum one of the RGB gray levels per pixel is assigned as the gray level in the white field. For a pure white image, this method displays the image only in the white field, thus CBU can be totally eliminated. For other colors, at least one gray level of color fields will be zero. Perception tests have confirmed that the color separation is markedly reduced.
6.1.2 The adaptive B/L for CBU optimization in laptop-sized LCDs
The consistent color orders of field such as CFA and RGBW methods can not fulfill all kinds of image for static CBU reduction, especially in cyan, yellow, and magenta images. Remaining two primary colors result in an obvious color separation during the observation.In order to overcome this restriction, we presented the intelligently adaptive backlight. According to the incoming video content, the exchange of RGBC/Y color sequence dynamically mitigates the CBU phenomenon to lighten the most sensitive green field. The redistribution of LC gray levels on these two color sequence is assessed by the gamma curve between gray levels and transmittances to maintain the white balance. We implemented the RGBC/Y method on a 32-in LCD platform. The brightness of a white image can reach at 400 nits at total power consumption of 50W, a half of power of typical CCFL backlight module [55]. From the CBU evaluation index, 27% to 56% of suppression ratio in the specific color pattern was obtained.
This redistributed method has been extended to determinate a suitable color in the fourth field. With RGBC/Y sequence, the fourth color is fixed as the cyan or yellow one, resulting in still one primary color with higher values of gray levels. An approach is to flash an adaptive image in a dominated field (D-field) and use the remaining fields to display the color residuals. Typical FSC displays with consistent color order like a rotating color wheel in a projector are unable to obtain an adaptive color field except the modulation of LED backlight.
The color backlight in D-field that induced the minimum color difference between CBU images and original ones was proposed for the CBU minimization. The simulation results of test images show that the 2x4 sampling period of image and 3-bit gray levels of color backlight can adequately simplify the calculation of color
difference, which is necessary in a real time application. Most importantly, an adaptive feedback control algorithm was described to locate the optimal D-field color efficiently. 8 sets of color backlight were evaluated at each frame and more accurate sets were used at following frame. Consequently, the proposed DRGB method concentrates the majority of image intensity into the D-field and owns the lowest color difference compared to those of typical RGB and RGBC/Y method. This average CBU suppression ratio around 70% of typical RGB driving is obtained. The color separation at the edges of objects in test images was essentially improved and agreed with the observation.
6.1.3 The recommended light profile for TV-sized FSC-LCDs
Adaptive dimmable backlights consisting of local addressable LEDs allow the D-field color to be optimum for each segment. For highly-colored images, the mass of image intensity in primary color fields may remain even through the entire backlight is modulated in the D-field. Nevertheless, color addressable backlights are capable to locally module the D-field along horizontal and vertical segments to resolve the issue of global adaptation, especially for large-sized LCDs. Moreover, these backlights generate only the amount of light required to correctly depict the video content while dimming underneath dark areas. The power consumption can be further reduced while simultaneously improving the black level [95]-[99].
To combine this highly-potential backlighting with FSC-LCDs, we have investigated the optical profiles on segments to influence the perceived image quality.
The brightness variation at the boundary between segments should be indistinguishable in the backlight image. The gradient of backlight image was used to evaluate the visibility of boundaries. According to contrast sensitivity function, we identified π/100 of background luminance as the threshold for the boundary
perception. Below this threshold, boundary was not sensible.
Several images with different spatial scales, ranging from coarse to fine detail, were examined. Considering the boundary perception and contrast enhancement, the size of 2 degrees was found to the optimal segment profile. The gradient is close to 100% below the boundary-free criterion. At a viewing distance of a factor of 3 longer than screen diagonal, the segment size of 2 degrees results in about 10x10 as the recommended and economical number of segments. For a closer viewing distance, more segment numbers are needed. In addition, the proper light profile in each segment was derived for high uniformity of three significant points. Considering the localized ability and contrast enhancement, the Gaussian function was the recommended profile of each segment in local adaptive backlights.
In conclusion, this dissertation explores the sequential driving and the evaluation of newly developed CBU reduction techniques. The practical circuit control and optical profile design has been examined on full-scale LCDs. In this thesis, we have demonstrated that the adaptation to image content has a great potential to attain high image quality and low power consumption, the essential capacities of FSC-LCDs.